1. Field
Embodiments of the present invention generally relate to substrate processing and, more particularly, to endpoint detection during etching of a substrate.
2. Description of the Related Art
A critical process in the fabrication of integrated circuits (ICs) is the removal of portions of various layers of materials formed on a substrate, such as a silicon wafer. Two of the most prevalent removal techniques are: 1) wet, or chemical etching, wherein a photoresist-patterned substrate is immersed in a chemical solution and 2) dry, or plasma etching, wherein a substrate is exposed to a bombardment of ions (e.g., a plasma of nitrogen, chlorine, and boron trichloride).
Plasma etching processes and apparatus are generally well-known for etching materials in semiconductor device fabrication. The process may begin with application of a masking material, such as photoresist, to a silicon wafer or other substrate. The masking pattern protects areas of the wafer from the etch process. The wafer may then be placed in a plasma reactor (“etcher”) etched after a plasma is ignited. This process is especially valuable for the definition of small geometries.
Irrespective of the particular etch process being employed, it is generally of universal concern to detect when an overlying material (“film”) has been completely removed (“cleared”). This is generally accomplished by detecting the presence of the underlying material, now exposed, in the plasma, such as by using an optical emissions spectroscopy (OES) technique.
With this technique, the OES equipment couples to a transparent window of a reaction chamber containing the plasma and monitors the optical emissions from the plasma. Conventionally, a single wavelength is extracted (filtered) from all of the emissions of the plasma, and the magnitude of the signal at the extracted wavelength indicates the present chemistry within the chamber. A change in this chemistry will increase or decrease the magnitude of the signal at the extracted wavelength and, as such, indicate a change in the process occurring within the chamber. For example, a plasma in a dry etch process is typically monitored by extracting the carbon monoxide (CO) emission line at 4835 Å. A change in the magnitude of the CO emission line is indicative of the endpoint of an oxide etch process.
Extracting a particular line, as well as suppressing the noise generated by the plasma at other wavelengths, requires substantial signal processing and careful OES equipment fabrication and calibration. Such endpoint detection requires the detection of a very small change in signal strength at a particular optical wavelength. As such, the desired signal is easily corrupted by noise. If care is not taken while monitoring the process, the endpoint may not be detected, and the wafer will be over etched. Such over etching may destroy the ICs on the wafer and reduce the yield of the semiconductor wafer processing system.
It is even more difficult to accurately monitor the etching process when the substrate being etched has a small open area between the etch-resistant features because the process signal from such a region is small relative to the process signal from other portions of the substrate. As used herein, the open area may be expressed as a percentage of the etch area to the surface area of the substrate. Conventional techniques that monitor the intensity of a particular emission wavelength are often limited to an open area of around 30% or larger. Smaller geometries in semiconductor processing may entail smaller etch open areas, even down to 1%, so conventional techniques may be inadequate.
Moreover, one type of signal processing technique used to detect etch endpoint involves neural principle component analysis (NPCA) known to those skilled in the art. However, NPCA requires lots of data and a corresponding amount of computation for endpoint detection, especially since the filter orders may be on the order of 20 to 100. Although processors are ever-increasing in speed, performing an NPCA may introduce significant delay between the actual occurrence of an event and the detection of that event, as much as a few seconds. During substrate processing where etching needs to be stopped immediately after the endpoint has occurred, such delay is unacceptable. Furthermore, the amount of computation required by NPCA monopolizes a substantial portion of the processor's power.
Accordingly, what is needed are techniques for a substrate processing system to precisely detect etch endpoint, especially for low open areas, without significant delay and with a lowered computation cost when compared to conventional endpoint detection techniques.